HiSORセミナー
"Heavy Fermions" as observed by photoemission and interpreted by the "PAM"
日時 2017年7月10日 （月） 14：35～
場所 放射光科学研究センター 2階 セミナー室
講師 Konrad Matho
(Institut Néel)
The general context of this seminar is the experimental and theoretical study of "Heavy Fermion" systems: Metallic compounds containing rare earth (RE) ions such as Ce and Yb. The experimental method considered is photoemission, as carried out either in angle integrated ("PES") or angle resolved ("ARPES") mode. The Periodic Anderson Model ("PAM") has frequently been employed to interpret the spectra. The subject, started half a century ago, is still surprisingly lively and full of new challenges.
I have collaborated with Clemens Laubschat's group at the TU Dresden in Germany. My theoretical contribution was to consult a PhD student in the group, Alla Chikina, in the development of a computer code for the PAM that uses our phenomenological Continued Fraction Method (CFM) [1]. The CFM was generalized from the Hubbard model to the PAM and the code was tested in comparison with results from Dynamical Mean Field Theory [2]. The benchmarking was carried out in the "Kondo Lattice" regime of the PAM, which is characterized by the presence of a "large" Fermi surface (FS). The physical meaning of this concept is explained in terms of Luttinger's counting principle [3].
The PAM englobes other scenarios, beyond the Kondo Lattice, such as charge transfer and mixed valent regimes. An overview of possible spectra is presented, as calculated with our code [4]. The PAM predicts damped van Hove singularities in the quasiparticle density of states. One of the new challenges is to demonstrate their presence in the PES data.
A detailed discussion of ARPES and PES results on YbRh2Si2 [5] and YbNiSn [6] uses an "asymmetric KL" scenario. We conclude that Doniach's KL model, based on a single Kramers doublet per RE ion, is not applicable. The excited 4f levels under the cristalline electric field exert a strong influence in stabilising the large FS at temperatures well beyond the Kondo temperature of a Kramers doublet.
[1] R. Hayn, P. Lombardo, and K. Matho, Phys. Rev. B 74, 205124 (2006)
[2] A. Benlagra, T. Pruschke and M. Vojta, Phys. Rev. B 84, 195141 (2011)
[3] R. M. Martin, Phys. Rev. Lett. 48, 362 (1982); J. Appl. Phys. 53, 2134 (1982)
[4] A. Chikina, PhD Thesis, Dresden (2016) and t.b.p.
[5] K. Kummer et al., Phys. Rev. X 5, 011028 (2015)
[6] A. Generalov et al., Phys. Rev. B (accepted May 2017)
問合せ先 島田賢也（放射光科学研究センター）
